One method of printing in different colors is to uniformly charge a charge retentive surface and then expose the surface to information to be reproduced in one color. This information is rendered visible using marking particles followed by the recharging of the charge retentive surface prior to a second exposure and development. This recharge/expose/and develop (REaD) process may be repeated to subsequently develop images of different colors in superimposed registration on the surface before the full color image is subsequently transferred to a support substrate. The different colors may be developed on the photoreceptor in an image on image development process, or a highlight color image development process (image next-to image). Each different image may be formed by using a single exposure device, e.g. ROS, where each subsequent color image is formed in a subsequent pass of the photoreceptor (multiple pass). Alternatively, each different color image may be formed by multiple exposure devices corresponding to each different color image, during a single revolution of the photoreceptor (single pass).
A major image quality drawback of xerography is "differential gloss", where the gloss of white or non-image areas (bare paper) is usually very different from that of fully-toned areas. This becomes particularly important in high-quality xerographic applications competing in a market accustomed to the look and feel of lithography. It is even more important for more demanding applications which require the look and feel of photography.
A solution to this problem involves the use of a "white printer", typically the addition of clear toner which has a gloss characteristic similar to the other toners. In one approach the entire page would be covered with clear toner, but this would not quite solve the differential gloss issue, since gloss is dependent on the local toner mass per unit area. Moreover, this would also further increase the toner pile height which in typical xerographic printers is already too large. Another approach involves the use of the image-wise deposition of clear toner, limiting it to the areas where there is no other toner present. This would solve the differential gloss problem, but requires a separate ROS (or another pass in multi-pass systems) and the creation of another separation, clear toner in addition to the normal CMYK separations, adding to computational cost.
The differential gloss characteristic of xerography is a major source of dissatisfaction in high-quality applications. Other image quality attributes of xerography have been greatly improved in recent years, but potential users accustomed to lithography typically object to xerographic images because of their differential gloss. Moreover, there is now considerable interest in pursuing photography-like applications using xerography. Ordinary xerographic images have been shown to look almost like photography, solely by providing a very uniform image gloss by placing the image behind a transparent film.
An object of the present invention is directed to a method to apply clear toner to achieve high-quality images at much lower cost and/or at higher speed than previously possible.